A vehicle may be propelled by an engine that imposes a reaction force (i.e., thrust) by expulsion of matter in a desired direction. Thrust is related to momentum of the expelled matter, which depends upon both the velocity at which matter is expelled and the mass of the expelled matter. To accelerate the matter for expulsion from the propulsion system requires input of energy. It has been proposed to transmit energy from an off-board source to an on-board propulsion system in the form of an electromagnetic beam. Off-board sources that have been proposed include remote facilities such as a ground-based emitter or array of emitters. Energy transfer to the propellant may be limited by temperature-handling capabilities of the propellant heater. This temperature limitation translates directly to a limitation on the efficiency of the engine such that increases in efficiency require advances in both microwave absorbing capabilities, and temperature and pressure handling capabilities, of the materials. Accordingly, it is desirable to have improved systems and methods for transferring energy from an external source to a stream of propellant matter in a propulsion system.
A thruster may use external electromagnetic energy to accelerate a flow of propellant and to generate thrust by expelling the accelerated propellant. In such systems, a flow of propellant may first be established in the thruster, followed by coupling of microwave energy to heat and accelerate the flowing propellant, thereby generating a flow of heated propellant. The flow of heated propellant may be used to produce useful work such as by being delivered to an exhaust nozzle for expansion (i.e., acceleration) and thereby generating thrust as the propellant is expelled from the nozzle. Accordingly, it is desirable for the use of an electromagnetic thruster that means for establishing a flow of propellant be provided in connection with the electromagnetic thruster.
In addition to establishing a flow of propellant, it is desirable that, for use of an electromagnetic thruster to propel a vehicle, or to otherwise produce useful work, the vehicle hosting the thruster be positioned to receive the beam of electromagnetic energy. Unfortunately, it may not be feasible, for a number of reasons, for such a beam to be directed toward a vehicle while the vehicle is positioned on the surface of the earth. As a result, it is contemplated that it may be necessary to direct the energy beam upwardly from a ground-based source away from the surface of the earth. Accordingly, it is desirable to provide for delivery of the vehicle into the energy beam (i.e., into a flight trajectory phase that intersects the upwardly-directed energy beam).
To satisfy this desire, some have proposed delivering the target vehicle into an upwardly-directed energy beam using conventional vehicle propulsion means, such as a solid rocket booster, implemented as a first stage for launch of the vehicle. Others have suggested releasing the target vehicle from another flying vehicle, such as an aircraft. Unfortunately, these proposed delivery methods involve the expense and complexity of the additional stage or vehicle, tending to mitigate some of the advantages of using external propulsion systems. Thus, it is desirable to have an improved system and method for delivering a vehicle into an upwardly-directed beam of electromagnetic energy. It would further be advantageous if the improved system and method for delivering a vehicle into a beam of electromagnetic energy could provide a single stage to orbit capability.
In addition to the above-described issues, experience with propellant heaters, such as those that have been proposed for use in connection with an electromagnetic thruster, indicates that an energy absorptivity of a material suitable for use in a propellant heater may depend upon a temperature of the particular material. As a result, as a temperature of the propellant heater varies, its energy absorptivity also varies, resulting in non-optimum energy absorption characteristics at some operating conditions. As one skilled in the art will appreciate, it is desirable that the propellant heater function so as to efficiently absorb and/or transmit energy throughout the entire range of temperatures at which the propellant heater is to be operated. To address this desire, some have proposed that a dopant be applied in a non-uniform manner to a ceramic (e.g., SiC) material in such a way as to produce a propellant heater that is specially-adapted to the expected temperature profile.
Unfortunately, such an approach is limited in that a propellant heater, through which propellant is flowing, may not exhibit a constant temperature profile. In use, the propellant heater may exhibit a first temperature profile when the propellant heater is first exposed to the electromagnetic beam. After propellant flow has been established, and after efficient coupling has been achieved between the flow of propellant and the propellant heater used in the thruster, the first temperature profile is likely to have changed. As a result, it may be difficult to simultaneously achieve delivery of the vehicle into the energy beam, establishment of a flow of propellant, and efficient coupling between the flow of propellant and the propellant heater used in the thruster.
Conventional propulsion systems may achieve thrust by combining oxidizer and a propellant, relying on an exothermic chemical reaction between oxidizer and propellant to produce heat. This chemical reaction often occurs when the oxidizer is flowing though the core of the combustion chamber where it is permitted to react with the solid or jet propellant that may be packed into the core of the combustion chamber. Unfortunately, such an approach may be limited in its efficiency and may involve chemicals (propellant and/or oxidizer) that are toxic. Still further, the exothermic reaction can be difficult to control and, in fact, may be explosive in nature.
Accordingly, it is desirable to have an improved hybrid thruster for an externally-powered propulsion system, particularly wherein the system avoids or mitigates some or all the above-described problems that are inherent in the prior art.